This invention relates generally to earth borehole apparatus and specifically to apparatus for centralizing well logging and/or completion equipment in the borehole.
Borehole centralizers are often used as a part of a string of well logging instruments traversed in a borehole by means of a cable. These centralizers have a plurality of contact members which are urged against the borehole walls, causing the centralizer body and the associated well logging instrumentation to be centered in the borehole.
Difficulty may occur when these centralizers are used with other borehole contact instruments, such as a caliper. A caliper is a device having a plurality of contact arms which contact the borehole sidewalls and by various means relate signals indicative of the size of the borehole. It is to be understood that when a multi-armed instrument such as a centralizer or caliper traverses an uncased borehole, grooves or tracks in the borehole sidewalls will typically result. When the arms of an instrument such as a caliper are located directly below the arms of a centralizer, the arms of the caliper will track the grooves made by the arms of the centralizer and therefore measure the indention or track in the borehole made by the centralizer arm rather than yielding a true reading of the borehole diameter. Therefore, it is desirable to be able to index or offset the centralizer arms from those of a caliper or similar instrument.
However, where tracking errors are not a problem, for example, as with instruments which do not contact the borehole, it is desirable for the centralizer arms to be able to freely rotate around the centralizer body and hence around the instrument string. This rotation is desirable because (a) it is desirable to allow any natural twist in the support cable to be released and (b) to prevent placing additional twist in the support cable due to the tendency of the centralizer arms to track spirally in the borehole. While it is possible to have a different centralizer for each purpose, it is desirable for reasons of both cost and efficiency to have one centralizer with contact arms capable either of free rotation or of being fixedly offset with relation to whatever other multi-armed instrument is being used.
Prior art in the well logging field has typically relied upon devices such as set screws or compressible collars on the contact arm carriers to secure an offset of arm indexing. These devices depend upon friction to maintin a position and do not provide a positive mechanical lock for the arm position. Therefore, changes in temperature, forces exerted upon the arms while traversing the well, or other borehole conditions could cause the arms to rotate away from their indexed position. Additionally, since the adjusting mechanisms are exposed to the borehole environment, they are subject to corrosion and fouling, thus tending further to decrease their effectiveness.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing a simple mechanical means for providing a positive mechanical lock on the rotation of the contact arms of a borehole centralizer or similar device while also allowing free rotation of such arms when desired.